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So far as has been observed, life does require liquid water. Some life can cease activity when dry and then restart when water resumes, but no life has been fou

by cjensen 12y ago

So far as has been observed, life does require liquid water. Some life can cease activity when dry and then restart when water resumes, but no life has been found that can live in an entirely dry environment. If this proves to be a requirement for life, that does validate the notion of a habitable zone.

mc808 12y ago
The liquid water requirement tells us a life-bearing planet can't be too hot, but finding life in the deepest parts of the ocean suggests that the water may not need to be on the surface. The nutrition raining down from the surface ultimately links back to photosynthesis, though, so it's not entirely convincing evidence. The "habitable zone" concept is still useful when it comes to actually searching for signs of life. I'm guessing a planet with organic life on the surface would have stronger signatures than one with a thriving ocean under 500 km of solid rock and ice.
- SideburnsOfDoom 12y ago
  > The nutrition raining down from the surface ultimately links back to photosynthesis This is a key point. The trench lives on the spillover from the surface. If there was no habitable surface full of life, there would be no life in the trench. Have a read of http://en.wikipedia.org/wiki/Whale_fall http://en.wikipedia.org/wiki/Whale_fall - the deep ocean floor may host ecosystems, but this does not mean that planets where the best conditions are like the deep ocean floor are "habitable". Similarly, the presence of mosses and lichens in Antarctic dry valleys is interesting, but does not prove beyond doubt that such organisms could evolve if the best conditions were like that.
ceejayoz 12y ago
> So far as has been observed, life does require liquid water. That's a bit like a newborn baby saying "so far as has been observed, there are only a couple people in existence".
- RickHull 12y ago
  That strikes me as a reasonable assumption to hold, pending evidence to the contrary.
  - ceejayoz 12y ago
    A baby doesn't have evidence that there's stuff outside his/her perceptions. We do - knowing that we keep finding extremophiles in hot springs and underground rocks and water in unexpected places like Mars and Europa, we should be a lot more careful than a baby when we say "this is how it is".
    - TeMPOraL 12y ago
      But then again, this is an universal counterargument and can be used to dismiss any kind of selection as hubris. We need to act to the best the information available allows us and learn from mistakes.
- KurtMueller 12y ago
  Thus is the nature of knowledge and of discovery? This is why the Scientific Method was invented, correct?
  - ceejayoz 12y ago
    Part of the scientific method is knowing when to say "we don't know yet". We don't know how to define "habitable" yet, so we should stop saying it.
    - vacri 12y ago
      You quoted "So far as has been observed". How much more hedging do you need? Basically you're arguing for something useless; that no definition be made until there is 100% certainty, regardless of caveats. Such a high bar gives us no utility.
IndianAstronaut 12y ago
But liquid water can be found in as far away places as Saturn's moon Enceladus and Jupiter's moon Europa.

by cjensen 12y ago

mc808 12y ago

The liquid water requirement tells us a life-bearing planet can't be too hot, but finding life in the deepest parts of the ocean suggests that the water may not need to be on the surface. The nutrition raining down from the surface ultimately links back to photosynthesis, though, so it's not entirely convincing evidence. The "habitable zone" concept is still useful when it comes to actually searching for signs of life. I'm guessing a planet with organic life on the surface would have stronger signatures than one with a thriving ocean under 500 km of solid rock and ice.

SideburnsOfDoom 12y ago
> The nutrition raining down from the surface ultimately links back to photosynthesis This is a key point. The trench lives on the spillover from the surface. If there was no habitable surface full of life, there would be no life in the trench. Have a read of http://en.wikipedia.org/wiki/Whale_fall http://en.wikipedia.org/wiki/Whale_fall - the deep ocean floor may host ecosystems, but this does not mean that planets where the best conditions are like the deep ocean floor are "habitable". Similarly, the presence of mosses and lichens in Antarctic dry valleys is interesting, but does not prove beyond doubt that such organisms could evolve if the best conditions were like that.

ceejayoz 12y ago

> So far as has been observed, life does require liquid water. That's a bit like a newborn baby saying "so far as has been observed, there are only a couple people in existence".

RickHull 12y ago
That strikes me as a reasonable assumption to hold, pending evidence to the contrary.
- ceejayoz 12y ago
  A baby doesn't have evidence that there's stuff outside his/her perceptions. We do - knowing that we keep finding extremophiles in hot springs and underground rocks and water in unexpected places like Mars and Europa, we should be a lot more careful than a baby when we say "this is how it is".
  - TeMPOraL 12y ago
    But then again, this is an universal counterargument and can be used to dismiss any kind of selection as hubris. We need to act to the best the information available allows us and learn from mistakes.
KurtMueller 12y ago
Thus is the nature of knowledge and of discovery? This is why the Scientific Method was invented, correct?
- ceejayoz 12y ago
  Part of the scientific method is knowing when to say "we don't know yet". We don't know how to define "habitable" yet, so we should stop saying it.
  - vacri 12y ago
    You quoted "So far as has been observed". How much more hedging do you need? Basically you're arguing for something useless; that no definition be made until there is 100% certainty, regardless of caveats. Such a high bar gives us no utility.

IndianAstronaut 12y ago

But liquid water can be found in as far away places as Saturn's moon Enceladus and Jupiter's moon Europa.